Subscriber signaling system



Oct. 20, 1953 R. H. HERRICK ETAL SUBSCRIBER SIGNALING SYSTEM original Filed Feb. 12, 1949 10 Sheets-Sheei'l l RoswELL H. HERRIcK ERwlN M. RoscHKE INVENTOR. ym M THEIR AGENT Oct. 20, 1953 R. H. HERRICK ErAL SUBSCRIBER SIGNALING SYSTEM l0 Sheets-Sheet 2 Original Filed Feb. l2,4 1949 Oct. 20, 1953 R. H. HERRICK ETAL 2,655,410

SUBSCRIBER SIGNALING SYSTEM Original Filed Feb. 12, 1949 l0 Sheets-Sheet 3 RoswELL H. HERRICK ER'waN M. RoscHK; INVENTOR.

Wm /v/ THEIR AGENT Oct. 20, 1953 y R. H. HERRICK ET AL 2,656,410

SUBSCRIBER SIGNALING SYSTEM Original Filed Feb. 12, 1949 l0 Sheets-Sheet 4 POSITIVE POLARITY FIG. 3

RoswELL H. HERRICK ERwlN M. RoscHKE INVENTOR.

THEIR AGE NT Oct. 20, 1953 R. H. HERRICK ETAL 2,656,410

SUBSCRIBER SIGNALING SYSTEM Original Filed Feb. 12, 1949 Sheets-Sheet 5 2 SMIISR FIG. 4

RoswELL H. HERRICK ERwlN M. RoscHKE INVENToR.

2q g THEIR AGENT o- Oct. 20, 1953 R. H. HERRICK ETAL SUBSCRIBER SIGNALING SYSTEM l0 Sheets-Sheet 6 Original Filed Feb. l2, 1949 III mh Roswr-:LL H. HERRIcK ERwlN M. RoscHKE IN VEN TOR. 311%@ M THEIR AGENT Oct. 20, 1953 R. H. HERRICK ETAL SUBSCRIBER sGNALING SYSTEM l0 Sheets-Sheet '7 Original Filed Feb. l2, 1949 "t N m .@E o H R. ...nnw Mmm T mw E ,2 J .l H.R.m.m

HM w. A

299535 .wL w m .mroz .EMS ww. o

H i Y V @m2 F B MUSE M ew. Eocsm am@ @E @d Si ...o lill |,m Etwz o no2 QNB/ l III' @Q FAW e H lll mmmzmomm, AW o lldm 595mg? 1 l o ESN@ 1I u D w- L Ulm CNM HHH om@ w @i R. H. HERRICK ET A1. 2,656,410

SUBSCRIBER SIGNALING SYSTEM l0 Sheets-Sheet 8 Oct. 20, 1953 original Filed Feb. 12, 1949 oct. 20, 1953 R. H. HERRICK ETAL SUBSCRIBER SIGNALING SYSTEM Original Filed Feb. l2, 1949 FIG. 7A

GENER l0 Sheets-Sheet 9 SWITCH OPERATOR SWITCH DEVICE RoswELx. H. HERRICK -ERwvlN M. RoscHKE JNVENTOR.

WMM

THEIR AGENT Oct. 20, 1953 R. H. HERRlcK ETAL 2,656,410

SUBSCRIBER SIGNALING SYSTEM Original Filed Feb. 12, 1949 l0 Sheets-Sheet l0 RECTIFIED EQUALIZER PULSES V ERT.

S YN CHF PULSES RoswELL H. HERRlcK ERwlN M RoscHKE Il n: INVENToR. [um

r THEM AGENT Patented Dot. 20, 1953 UNE'E'E STATES 'TENT OFFICE SUBSCBIBER SIGNALING SYSTEM Roswell H. Herrick, Lora-in, Ohio, and Erwin M. Roschke, Broadview, Ill., assignors to 'Zenith Radio Corporation, a corporation of Illinois 9 Claims.

This application is a division of copending application `Serial Number 75,988, filed February 12, 1949, entitled Subscriber Signalling System, by R. H. Herrick et al. and assigned to the present assignee.

rThis invention relates to coded electrical signailing systems of the subscription type, and more particularly to such systems in which coded electrical signals are transmitted on one channel, and key signals for decoding the electrical signals are transmitted solely to authorized subscriber receivers on a second channel.

The accompanying drawings and associated text describe the technical details and method of operation of a subscription type of television system embodying the instant invention. However, it may be helpful to have as background information a simplified explanation of the underlying principles of that system to facilitate a quick understanding of the illustrated arrangements.

In accordance With one form of the invention, a television transmitter is provided with a coding arrangement which, in response to an applied signal, is capable of changing the operation of the transmitter from one mode to another, the change in modes constituting a coding function to introduce an aspect of privacy by virtue of which unauthorized receivers are not able to utilize the transmitted program signal. A control signal which may have a random amplitude is established in a suitable vehicle, such as a tape, a record disc, a memory tube, or the like, so that a scanning element or other pick-ofi" device may recurrently derive the recorded or stored signal. This derived signal is applied to the coding arrangement to effect a change from one mode of yoperation to another in the transmitter in a prescribed ceding schedule represented by the amplitude variations of the control signal. Preferably, the cycle of the coding schedule is very short with respect to any program period. Hence, the transmitted signal is coded in accordance with a repeating coding schedule many times in a given program.

Obviously, it is necessary that subscriber receivers be supplied with a key to enable their utilization of the otherwise unintelligible transmitted signal. To that end, it is contemplated in the present invention that the subscriber place a call to an exchange associated with the aforedescribed transmitter. in practical utilization of the system, this call is made prior to the program interval in which the subscriber is interested. During the time that the line connection is completed for the call, the control signal of the transmitter derived by scanning the record or storage device as indicated above is delivered over the line circuit to the receiver Where plementary to the coding arrangement of the Y transmitter and which is subject to the influence of a key signal to decode the received but scrambled transmission. A key signal, for application to the decoding arrangement, is obtained by scanl ning the record or storage unit in the receiver. Thus, considering the receiver alone, its decoding arrangement receives a key signal that represents the coding schedule of the scrambled transmission and responds to that key signal to effect compensating changes in the mode of operation to accomplish decoding. The decoding thus obtained permits the receiver to utilize the received program signal and reproduce intelligable images.

For cooperative action on the part of the transmitter and receiver, it is necessary that their operations be precisely correlated. It has already be realized.

` simple and may be satisfied by synchronizing the been explained that the coding at the transmitter and the decoding at the receiver are fully complementary but synchronizing and phasing must also The synchronizing is essentially scanning at the transmitter and receiver to operate at identical and synchronous speeds. Phasing is attained by comparing the time relation of corresponding portions of the coding cycles at the transmitter and receiver. For that purpose,

' a special phasing signal may be sent out from the transmitter at the start of a coding cycle and preferably during a i'leld retrace interval so that it has no adverse effect on the picture informac tion. Likewise, a phasing signal may be provided in the record or storage device at the receiver to represent the start of its coding cycle. These phasing signals may be applied to what is essentially a phase detector Which provides a control effect in accordance With the time relation of the compared phase signals in. any coding cycle.

'- That control effect may adjust the scanning at the receiver to maintain proper phase as Well as synchronizing relations.

' whereas unauthorized receivers experience Wholly unintelligible reception of such programs.

Subscription type signalling systems have been proposed wherein television signals are radiated in coded form, and key signals for decoding the coded signals are transmitted to authorized subscriber receivers over suitable line circuits, such as the telephone lines. They key signals are supplied to individual .subscribers upon request and a suitable charge is made for the use thereof. Such systems, as heretofore proposed, require the continuous use of the telephone lines as a transmission link for the key signals, and have-been objected to on the ground that they unduly inter.-

fere with the normal service voi? `.the telephone sys tem.

The present invention provides a subscription type of signalling system in' which coded signals are transmitted to subscriber receivers, and vkey signal information required to enable the receivers to decode the coded signals for along period of time is supplied thereto over the telephone lines, but in an extremely short time interval and by means of an essentially normal telephone call. In this manner, continuous use of the telephone lines for supplying key signals throughout entire program intervals is avoided, and no ob-.

jection may be .raised to the eifect that the system interferes with the normal service of Ithe telephone line.

It is, therefore, an object of this invention to provide a coded signalling system of the subscription type in which key signals for decoding the coded signals ar-e produced and distributed to subscriber receivers in an improved and more convenient manner than in previous systems of this type.

Another object of this invention is to provide such a system in which sufcent key signals for decoding the coded program signals for a relatively long period of time arel produced and distributed to individual lsubscriber receivers ina relatively short time interval, preferably, before th-e transmission of the coded program signals, whereby more efficient use isY made of the key signal channel, that is, this channel is not used continuously by an individual subscriber but for short time intervals only, leaving the channel free for other purposes and for use by other subscribers during periods when it is not in use by the first mentioned subscriber.

Yet another object of this invention is to provide such a system in which use may be made of telephone networks to distribute the key signal to subscriber receivers, whereby each subscriber uses the telephone network for an eX- tremely short time interval to obtain sumcient key signals to decodethe received coded signals for a program that mayextend over a relatively long period of time.

Another object of this invention is to provide a transmitter for use in such a coded signalling system, in which transmitter a pattern of key lsignals is produced for recurrently coding the transmitted signal and for distribution to subscriber receivers.

A further object of this invention is to provide a receiver for use in such a coded signalling system, which receiver is-capable upon the receipt of a key-signal pattern to utilize this pattern recurrently to decode the received coded signals.

r)The features of this invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by referconjunction with the accompanying drawings, in which:

Figure 1 shows a transmitter which incorporates the present invention,

Figure 1A shows a modification of a portion of the transmitter of Figure 1,

Figure 2 shows diagrams useful in the understanding of the operation of the transmitter of 4Figure l,

Figures 3 and 4 show in detail the circuits of certain components of the transmitter of Figurev 1,

Figure 5 shows 'diagrams useful in the understanding of the operation of the circuit of Figure 4,

Fig-ure 6 shows a convenient system for use as a central distributing station,

Figure 7 shows a receiver for operation in conjunction with the systems of Figures l and 6,

Figure 7A shows a modification of a portion of the receiver of Figure 7, and

Figure 8 shows in detail the circuit of one of the components of the receiver of Figure 7.

`Copending application Serial No. 742,374, filed April 18, 1947, entitled Radio Wire Signalling System by Alexander Ellett et al., which has now issued as Patent 2,510,046, May 30, 1950,' and copending. application Serial No. 773,848, filed September 13, 1947, entitled` Image Transmission System by Erwin M. Roschke, now issued as Patent 2,547,598, April 3., 1951, both assigned to the present assignee, disclose typical coded electrical signalling systems of the subscription type. In these systems, coded program signals are radiated and key signals for decoding the coded signals are distributed to subscriber receivers over suitable wire line conductors. In such systems the key signals are transmitted continuously over these conductors to the subscriber receivers throughout the entire program period. In accordance with the present invention, sufficient key signal information for decoding the coded electrical signals for long periods of time are transmitted tothe subscriber receivers in a brief time interval, and not continuously. Hence, when a telephone network is used for the distribution of the key signals, a subscriber may obtain sufficient information by means of a short telephone call, to enable his receiver to decode the coded signals for a long program interval, for example, for several hours.

The present invention willbe described as applied to a coded subscription signalling system, such as the system disclosed in application Serial However, it is to be understood that the invention may similarly be applied to other types of such systems, such as those disclosed in application Serial No. 742,374. The Systems 0f Figures 1 and7 vare similar in many respects to the system shown in application 773,848 and many of the components of the systems of Figures I and 7 have been described in detail in that application. Since these components in themselves form no part of the present invention, a detailed description thereof is believed to be un- TleCeSSaI'Y.

Referring' now to Figure 1, the transmitter there represented schematically includes a camera l having mounted therein a television picture converting device 2 of the iconoscope, image orthicon, or any other suitable type.- The camera l .includes a lens system 3 for focusing images of scanned subjects on the device 2. The output of device 2 is coupled to a video amplifier 4 which in ence to the following description when taken inA g5 turn is coupled to a synchronizing signal and pedestal mixer amplier 5. The mixer amplier 5 is coupled through a background reinsertion device 6 to a carrier wave generator and modulator 'I, and the modulator may be coupled to any suitable type of antenna 8. Vertical and horizontal synchronizing pulses and blanking pedestals are generated in the system by means of a generator 9, this generator being coupled to the mixer amplier 5 by way of leads I0 and to a vertical sweep generator II by way of leads I2. The generator 9 is also coupled to a horizontal sweep generator I3 through a delay line and switch I4. The vertical and horizontal sweep generators are respectively coupled to the vertical sweep coils I5 and to the horizontal sweep coils I6 of device 2. The generator 9 is further coupled to a random frequency divider II which may be of the type disclosed and claimed in copending application Serial No. 32,457, Erwin M. Roschke, entitled Random Frequency Divider, led June 1l, 1948, now issued as Patent 2,588,413, March 1l, 1952, and assigned to the present assignee. The divider I'I is connected through a switching device I8 to a key signal oscillator I9. Generator 9 is also coupled to a key signal lter and switch operator 2i? by way of leads 2 I, the switch operator ZIJ being connected to the delay line and switch I 4 by means of three leads 22.

In the operation of the system thus far described, video-frequency signals representing a scanned object are produced by the device 2. These signals are amplified in the video amplifier Ii and the amplified signals aremixed in the mixer vamplifier 5 with vertical and horizontal synchronizing pulses and blanking pedestals from the generator 9. The resulting composite television signal from mixer amplifier 5 is passed through the background reinsertion device 6, and this signal, properly adjusted as to background level, is modulated on a suitable carrier wave in I- stage 7, the modulated carrier wave ybeing radiated from the antenna 8. The vertical and horizontal scannings of the device 2 are controlled respectively by the sweep generators I I and I3. The vertical sweep generator II is synchronized directly by generator 9 through leads I2, and the horizontal sweep generator I3 is synchronized by generator 9 through the delay line and switch I4. The delay line and switch I4 has two positions, and when in one of these positions this stage passes the horizontal synchronizing pulses directly to the generator I3, and when in the other of these positions these pulses are delayed a certain time interval. The delay line and switch I4 is switched from one position to the other by the switch operator 2e through leads 22, and this switching at spaced time intervals causes the horizontal synchronizing pulses passing therethrough to be delayed during such intervals. This delay causes the generator I3 to delay the timing of the horizontal scanning 0f device 2 and, hence, during these spaced time intervals the video-signel components of the radiated television signal are delayed with respect to the synchronizingsignal components thereof, thus effectively coding the radiated signal.

Vertical synchronizing pulses from the generator 9 are randomly frequency divided in the frequency divider I'I, and when the switching device I8 is closed these pulses cause the key signal oscillator I9 to generate bursts of key signal, each burst corresponding to a pulse from divider I'I. In a manner to be described, these bursts are recorded for a predetermined interval to form a key Vsignal pattern in a recording device, or other suit- `able signal storage device, and this pattern from the recording device is impressed recurrently as a 'coding signal on the switch operator 20 over the leads 23 which connect the recording device to the switch operator. The key signal lter and switch operator 20, in response to the coincidence of a burst of key signal on the leads 23 and a ver- -tical synchronizing pulse on the leads 2 I, actuates the delay line and switch It, such actuation thereby commencing during vertical retrace intervals `of the device 2. In this manner, changes in the television signal from a normal mode to a mode in which the video signal is delayed with respect to the synchronizing signal due to actuation of the delay line and switch I Il take place during these vertical retrace intervals and distortion in "the image reproduced in subscriber receivers is `thereby avoided.

` A detailed description of the above described type of coding system is given in application Serial No. 773,848, together with a description of the circuits and operation of the various components, and further description herein is believed to =be unnecessary. As previously stated, the present system differs from the system of the above application in that signals from oscillator I9 are not impressed continuously on the switch operator 20 over the leads 23, but these key signals are recorded for a certain time interval to obtain a `key signal pattern, and this pattern is impressed recurrently as a coding signal on the switch operator 20 over the leads 23 during the program interval when the coded television signal is radiated from antenna 8.

The term key signal pattern is used to refer to a number of key signal bursts from the key signal oscillator I9 which are recorded and used recurrently as a coding signal to effect coding of the television signal.

The following is a detailed description of the means whereby the key signals from oscillator I9 are recorded to obtain the key signal pattern.

Vertical synchronizing signals from the generator 9 are used to synchronize the generator 24 to -the frequency of these synchronizing signals, or

some harmonic thereof, and the generator 2e .drives a motor 25. This motor, in turn, drives a recorder which is shown to comprise endless magnetic tapes 2t, 2l and 28. These tapes, and 'others to be described herein are, preferably, ro-

tated by the driving disks by means of sprockets .such as the film in motion picture projectors to .insure that there is no slippage. Furthermore, the illustrated recorders may be replaced by any known types of signal storage device, such as 'memory tubes, disk or wire recorders, and the like The pulses from the frequency divider I1 are passed through the switching device I8 to the key signal oscillator I9. Switching device IS is arranged, in a manner to be described, so that `when it is closed, pulses from the frequency divider I'I energize the oscillator i9, and generated oscillations are recorded on the tape 2l' through a recorder head 2e, the tape 21 being designated as the code tape. Switching device i8 is so controiled that it remains closed for not more than lone revolution of the code tape 2l `and a pattern of key signals from oscillator I9 is thereby recorded on this tape. The key signal pattern recorded on tape 2l is used recurrently f to actuate switch operator 20, this pattern being impressed on the switch operator over leads 23, which leads extend to the operator from a pickup head 30 associated with the code tape 21. The pattern of key signals from the pick-up head1-Baisfurther. impressed through anamplifierv 3l:.onconductors 32, forming a linecircuit leading to a'` central distributing station, to be described. Conductors :t2-may bey any form of a` link: between the transmitter and the dis- 'tributing station, and, when desired, the patternof key signals-may be radiated to this station from the transmitter, thus dispensing with these conductors.v

ltfisnecessary.` thatswitching, device I8 close andopenn an interval betweenthe pulses from frequency divider-iii; and also that this switching' device remain closed for no more than one :revolution of` the code tape 21. To accomplish thisytape 28 which is designated the switch tape is rotated by motor 2 5 ata speed much slower, than tapes Ziiiand 21, by means of a reduction gear 33, tapes and 21 rotating preferablyat the saine speed.` For example, when the Atapes 2,6'and21: are rotated at the preferred speed. oaf-.69,revolutions per minute, tape 2,3 may be rotated .at one revolution per minute. A switching Asignal having a frequency, for examplaof 35500 eyclesper second'is recorded'by any known means on the switch tape 2.8, and this` tape is adjusted :sd that this switchsignal is phased With respect .tothe vertical synchronizing signals of the system to, close the switching device I8 in an interva-l between succeeding pulses from the frequency divider, l-l'. rlhe duration of the switching .signal recorded onv the switch tape 2t is made .suchthat when this, signall is impressed on the switching device i8., this` device is closed for one revolution only of the code tape 2.1. Therefore,

Awhen-it is desiredto recordv a key signal pattern on the code tape 21, or to change an existing pattern on this tape, a switch 3.4i in the leads connecting an erasing signal source to the erasing head 31 associated with the code tape 2li is vclosed forseveral seconds in an interval before the leading edge ofthe switch signal on the switch tape 28 reaches the pick-up head 35 associated with the switch tape. Closing the switch 3d applies an erasing signal from the source 36 to the erasing head 31, and an existing signal on the code tape t1v is thereby erased. Switchv 34 is then opened and a switch 38 in the leads connecting the pick-up head 35 to the switch device I8 is closed. Now, when the switch- .ing signal on theswitch tape 28 reaches the pickup head 35, the switch device I8 closes and passes pulses from the divider i1 to the key signal oscillator |79. The switch device IB is maintained closed by the switch signal from the switch tape 2.8 for one revolution of the code tape '21, and -during' this revolution of the code tape a new keysignal pattern is recorded thereon. When the code tape 21 has completed the revolution, the trailing edge of the switch signal recorded on tape 23 passes the pick-up head 35, and the switch device I8 opens. This new key signal pattern may thereafter be picked up by the pickuphead 3U associated with the code tape 21, and applied recurrently to the switch operator 2G over the leads 23 to actuate this operator, as previously described.

Therefore, in the system thus far described, a coded composite television signal is radiated from antenna 8 in Which, during spaced time intervals, the video-frequency components are delayed a certain amount with respect to the synchronizing-signal components of this composite signal, and in which the change from a normal mode of the television signal to a mode in'wliich the video-frequency components are so delayed takes place during vertical retrace intervals. This coding of the television signal is effected bythe switch opera-tor .Zllin response to the coincidence of vertical synchronizing Ypulses on leads 2l, and respective key signal bursts of the key signal pattern on leads 23. The described system also provides` for the transmission of the key signal pattern to subscriber receivers over the line circuit 32 and through a central distributing station.

The tape 26 is designated the control tape, and as previously mentioned, this tape is preferably driven` at the same rate of speed as t-he code tape 21. The purpose 'of the control tape 2G is to provide a control signal so that a decoding tape at a subscriber receiver may be synchronized with the code tape 21, in a manner to be described. The control signal is recorded by any known means on the lcontrol tape 25,;and the control tape is adjusted so that this control signal is phased to indicate preferably the point on the code tape 2'! of the commencement and termination of the key signal pattern recorded on the last mentioned tape. The control signal recorded on the control tape 26 is given a frequency substantially different than the frequency of the key signal bursts recorded on the code tape 21 and the control signal is transmitted to subscriber receivers with the key signal over the line circuit 32 and through the central distributing station.

The control signal recorded on the control tape 26V is also used to cause a second control signal to be transmitted to the subscriber receivers over the same channel as the coded television signals. In a manner to be described, the subscriber receivers utilize the time coincidence of the two control signals to effect synchronism between decoding tapes at the receivers and the code tape 21. As stated, the first control signal is transmitted to the subscriber receivers over the line `circuit 32, this signal being picked-up from the control tape 26 by a pick-up head 40 associated with this tape andV amplified in an amplier 39 that is sharply tuned to the frequency of this signal and has its output terminals coupled to the line circuit 32 to apply this control signal thereto. Furthermore, the control signal from the pick-,up head 40 is impressed o n a control signal phasing circuit 4| over the leads 42. The circuit 4I acts in a manner to be described, to pass at proper intervals vertical equalizer pulses from generator 9 to the mixer amplier 5, these pulses having been frequency multiplied in a frequency multiplier 43. The pulses passed by the circuit 4i form the second control signal and this signal is transmitted to subscriber receivers over the television signal channel.

In the embodiment of Figure l, the various operations of erasing a key signal pattern from the code tape 21, and recording a new pattern thereon are preferably completed before actual transmission of the television program by the system. It is contemplated in this embodiment that a key signal pattern on the code tape 21 be used recurrently to code the television signal for an interval corresponding to one or more television programs, for example, for one or two hours. After this program interval, the pattern vis changed and a new pattern recorded on the code tape 21 for efecting coding during the next succeeding program interval. Hence, it is necessary to disable the transmitter between such program intervals, so that an existing key signal pattern may be erased and a new pattern recorded on the code tape 21.

Figure 1A shows a modification of the system of Figure 1, and in the modified system a plurality of code tapes 21a, 21h, 21o each have a different key signal pattern recorded thereon. The different key signal patterns may be recorded on the respective code tapes by means of the apparatus shown and described in the system of Figure 1, this recording being effected before actual transmission of the television signal, that is before any of the program intervals.

Amplifiers 31a, 31h, 3io have input terminals connected respectively to the pick-up heads 36a, 30h, 30e associated respectively lwith the code tapes 21a, 2117, 21e; and the output terminals of these ampliers are connected to respective line circuits 32a, 32h, 32e extending to the central station (not shown) The key signal patterns on the code tapes are amplified in the respective ampliers and are transmitted over the line circuits to the central station. For a certain program interval, for example, for the first television program, a switch lilia is closed, and the pattern from the code tape 21a is used to code the television signal during the interval of this program. At the end of this program, switch lisa may be opened and switch lib closed, and the pattern from code tape 21h used to code the television signal for the next succeeding program interval. Similarly, the pattern on the code tape 21e may be selected and used to code the television signal for an ensuing program interval. In this manner the coding of the television signal may be changed for each program or group of programs, without the need of disabling the transmitter to erase an existing signal pattern from the code tape and to record a new pattern thereon. It will be understood that the selector switches 44a, Mb, 44o may be actuated by any well known form of time control apparatus to accomplish a change from one code tape to another automatically and at preselected time intervals.

Figure 2 shows graphically the various signals recorded on the tapes driven by the motor 25 of Figure 1, and the relation of these signals to each other, to the vertical synchronizing pulses from generator 9, and to the pulses from random frequency divider l1. Referring now to Figure 2, the vertical synchronizing pulses are sho-wn in curve 2A, and these pulses occur at the indicated intervals. rfhe vertical synchronizing pulses of curve 2A are impressed on random frequency divider l1 of Figure 1, and this divider acts, as described in previously mentioned copending application Serial No. 32,457, to select at random, certain of the vertical pulses, such selected pulses being shown in curve 2B. The signal recorded on control tape 26 of Figure 1 is illustrated in curve 2C. This signal may conveniently have a frequency of 3,000 cycles per second, and is so phased that the leading edge thereof reaches head il of Figure 1 and is picked-up by this head in the interval between two vertical synchronizing pulses, such as pulses 45 and 4B of curve 2A. The control signal continues to be picked-up by head i0 up to the interval between pulses @t and 41 of curve 2A. The switching signal recorded on tape 28 of Figure 1 has any suitable frequency, and the tape 28 is adjusted so that this switching signal is phased to be rst picked-up by head 35 and applied to switch- .ing device I8 at the same instant that the con- 40; assuming that switch 38 is closed. The switching signal is shown in curve 2D and continues for a complete revolution of the control tape 26 and the code tape 21, these latter two tapes rotating at identical speeds. Hence, Switching device I8 is closed in the interval between the vertical synchronizing pulses i5 and 4B, remains Iclosed for a complete revolution of the code tape, and is opened by the termination of the switching signal of curve 2D, the trailing edge of the switching signal passing the head 35 in the interval between vertical synchronizing pulse #it and the next succeeding vertical pulse. When switching device iii is closed, the pulses of curve 2B are impressed on key signal oscillator I9 and a burst of key signal from this oscillator is recorded on code tape 21 in response to each pulse. Hence, pulses 49-53 of curve 2B cause bursts of key signal Sli-58 of curve 2E respectively to be recorded on the code tape 21, the burst 58 occurring just before the switch signal of curve 2D causes switching device I8 to open, sufficient time being allowed so that the entire burst 58 is recorded on the code tape 21. The bursts 54-58 recorded on the code tape constitute the key signal pattern and as previously described, switch 38 is then opened, and this pattern is used recurrently to actuate the filter and switch operator 2e of Figure 1 until such time that it is desired to change the coding pattern of the system.

The switching device I3 is shown in detail in Figure 3. The switch signal from pick-up head S5 of switch tape 28 of Figure l is impressed across terminals 59, one of the terminals 5S being grounded and the other being coupled to a control electrode B0 of an electron discharge device Si through a capacitor 62. The control elec trode Sii of device 6i is connected to ground through a grid leak resistor et, and the cathode Se of this device is connected directly to ground. The anode 65 of device Si is connected to a source 50 of unidirectional potential through a tuned circuit 61, the negative terminal of source @t being grounded. The tuned circuit 61 consists of an inductor G8 shunted by a capacitor 69, this circuit being sharply tuned to be resonant at the frequency of the switching signal from tape 2B of Figure l. The circuit of device Si amplies the switching signal, and discriminates against signals having frequencies other than the frequency of this switching signal. The amplified switching signal from device tl is rectified by a rectifying device 10, this rectifying device having a cathode connected to anode G5 of device 6I, and an anode connected to a control electrode 1I of an electron discharge device 12. Control electrode 1! of discharge device 12 is connected to the common liunction of series connected resistors 13 and 112, these resistors being connected across source t6 as shown. The anode of device 10 is coupled to ground through capacitor 15. The anode 16 of discharge device 12 is connected to the positive terminal of source 65, and the cathode 11 of this device is connected to the cathode 18 of a discharge device i9, the cathodes 11 and 18 being connected to ground through a common resistor te. The device 12 is so arranged that in the absence of the switching signal across terminals 59, this device is highn ly conductive due to the positive bias on its control electrode 1l from the potential divider action of resistors 13 and 1li. The space current flowing in device 12 causes a current i'iow through common cathode resistor 8U, hence increasing the ifjnpressed across termi h yseg rt 'rn'inals being grounded i ooupledto 'control electrode Sothr righe ycapacitor g 33, andl cont coizge'cted to ground i e A ressorts i I,

terminals 59, the pulses atr terrninalsvi are aniiln edgloy` device "i9,` and 'these ampli'ed pulses -are yob'tailie-d across vterminalsi3?to actuate lfzey signal oscillator i9 during spaced operating intervals.

lThe 'control signal phasing circuitfi 4 of Figure :A15 "622er `device "i"|S,' and the c'athode'of device '|21 f1 .is shownl ,in ldetail* injriguie e, and diagrams i explaining itsoperation arejshovrn in Figure 5. A'Inv thel circuit 'of Figure 4, verti l blah-king pulses from generator 9 of Eigure'l ar impressed across terrninals 88V, these pulses ha ngja 'f orrn such a'vsshown in]` curve 15A. Thelolanking ypuises impressed across terminals are diileren'tiated in `a 'differentiating circuit which includesga capacim ipi559:,and 'e registeren, thefdiiiejrfeiiiiates puise@ having the forrn shov/ninjcurve-E'B. Thesedifpositiver lignei 12'@ catiiede i2! of device! is is connected te ground.

12 sist' I3. 'THe 'anode iis oi device mi fis-'cone' nected to the positive vterninal of source |'3I throughurtleifWinding H5 of transformer |33. The positive peaks of the differentiated 'pulses 5 shown `iiifc'u'rife 5D, which peaks correspondto [5 of the terminals lit is rgrounded"and the other Vis coupled to the control electrode H'l of an elecv,tron discharge `device 'H3 through a capacitor -'||9, control electrode being connected to ground througha grid leak resistor |2Gi. IThe and anode |22 of this device is connected tothe positive terminal'o'f a'so'u'rce |23 4of unidirectional potential through a tuned circuit |2i, consisting of an 'inductor "i525 shunted by a capacitor In '9.5 |25, and the negative terminal of source '|253 is grounded. Th'ecircuit I2@ is tuned to be "reso- 'nant'atthe frequency of the'control signal im'- Vpressed across ltelfnirals ||,`and the circuit of device 'Ht ampliiies thiscontrolsignal anddiso oriininates"against fsignals `having frequencies other than that* of th'e'- control signal.

The-'amplified control signal `fromdevice 1I |8 is rectiedby'mfe'ansof a rectifying'deviceH'I. Theano'df'e of device |2'i is" connected-to the anode is'cou'pledtofgound through Aa capacitor |128 and a shuntconnectedresistor |29. A positiveunidirectional'potential, as shown in` curveBI-I, ap- -pe'arsjacross resistor "129 in response to the 'am- 4-0 `pliiled control signalfand this potential is "applied v'to 4the controliele'ctrode c|3llor 'an electrondis- Charge device `v|'3|. Cathodeilt'? of ldevice |`3| is connected `toilground through a resistor |33 shunted by a capacitor |3t. Cathode |32 is fur- "therfconniect'ed"tol ther positive terminal 'of source iig .ostinata anni., L The Gained@ @i of this-device is connectedto ground11,;h'rou'gl'i'a cathode resistor 98 shu'ntedby'a capacitor 99. The' anode A. r B of device'QfZ is connected to the positive termi- `5 nal of a'source ltlfof unidirectional potential.

A through a further Winding |92 *of `v transformer "negativepolarity appear at thenanode Ii'ilor" 'de-,4

Vice 912,1and'the `blocking`oscillatorisso adjusted 6 that these-[pulses have the orm'andduration shown in curve 15C.Vv The -pulse output of the rst blocking oscillator is 'differentiated by a diierentiating circuit consisting of fa'capacitoro mi and 'a resistor 'm5,' arie thedineren-dateci'`65 pulses,"shown in curve 5D, are'used to trigger a second blocking oscillator. The' diierentiated y pulses are impressedon the control electrode it of' an electrondischargedevice' |531 through a Winding |88 of'a transformer '|89 and through a couplingI capacitor IIE, controlele'ctrode it@ being connected to'g'round through an adjustable resistori |'I. The cathode |'|2 of Adevice |01 `por'sitivel'y biasedby theiootential divider action of'resistr |35'i1and-re`sistor |33. 'Screen electrode *|7355 isoonnectedtothepositive termin-alof source |23'through Vva, resistor, and this felectrodeiis ,Ehe'discharge "o iooup'led"tofgrourid'through'a by-pas's capacitor YA`|33. i; Pulses from the "fsecond blocking oscillator, Yshown' in c'urVefEE,1 are :impressed-"on af second control 'electrode i352 'fof device |3I. from cathode D 'r||2""offfdevicel'fil tl'irougha couplingcaipacitor M u, this control f'eie'etrode being eo'iinecte'a'to fgroundfthreugiilatei-idileak resistor |41. 4'Devi'fiie |'3'| 'is-'sobiased'that this device becomesconduc- "tive *only at fs'uch'ffintervals 'when the f rectified -control-"signalgas 'shown 'in' curve'C 5H,l is impressed "OrrCOntl'Ol' 'lectrde |30' at the Same time vtl'l'ett "the-'pulses'g'ls'hown in curve SEQ are impressedfn Y- control electrodel 39. f-At such intervals'4 of signal coincidence, thel potential ofanode |42 dropsdue 1:10 the current "new through load resistor |213, 'Which'rrfes'istor corineotsthis'ande to the Iiosi- -tive terriiin'alo soir-'roe |23.

The' anode maior device 'i 3 r 'is cupiedr-toithe "-eontroi eieetrodeifmi ef fan eiectron discharge u'eviceims through e;bowling-capacitor il 4s the 'control f'elec'trde TQQ being connected to ground through e gridieak' resisten a1. "TheA snede' 4ma -'oif'= i'evieeY m5 fis '-feoniiecteq to the positive Jaer- `?3,- andthe control*electrode |44 of this device is also connected to this positive terminal through a resistor |49. The cathode |56 of vdevice |45 is connected to the cathode of an electron discharge device |52, and these cathodes are connected to ground through a common cathode resistor |53. When discharge device |31 is in its non-conductive state and the anode |52 of this device is at a relatively high potential, the control electrode |44 of device |45 is biased positively due to the potential divider action of resistors |49 and |41, and under these circumstances a large amount of space current nows in device |45. This flow of space current causes the discharge device |52 to be non-conductive due to the potential drop across the common cathode resistor |53. However, when the anode |42 of device |3| drops to a relatively low potential value due to the conductivity of device |3|, the positive bias on control electrode |44 of device |45 is reduced and the resulting decrease in space current through device |45 causes the device |52 to become conductive to positive signals impressed on control electrode |54. Hence, device |52 is conditioned to repeat positive pulses applied to control electrode |54, whenever the rectiiied control signal is impressed on control electrode |36 of device |'3| simultaneously with the application of a pulse from the second blocking oscillator on control electrode |36 of this device.

Vertical synchronizing signal equalizer pulses are obtained from generator 9 and these pulses are frequency multiplied in multiplier 43 of Figure l to a frequency of 120 kilocycles per second, for example. equalizer pulses, shown in curve 5F, are impressed continuously across terminal |51, one of these termina-l5 being grounded and the other being connected to control electrode |54 of device |52` The anode |55 of device |52 is connected to the positive terminal of source |23 through a load resistor |56 and a pair of output terminals 215 are connected to this anode and to ground. Device |53 repeats positive pulses applied to control electrode |54 only during such intervals when the control signal shown in curve 5G, and the pulse output of the second blocking oscillator shown in curve 5E, occur simultaneously, and during such intervals a burst of the multiplied equalizer pulses, shown in curve 5J, appears across output terminals 215.

Hence, it can be seen that the control signal from control tape 26 of Figure 1 is transmitted over conductors 32, and occurs once per revolution of the control tape and code tape, and lasts for a short interval. This control signal furthermore causes a burst of signal composed of multiplied equalizer pulses to be radiated on the video carrier once per revolution of these tapes. The phasing circuit 4| of Figures l and 4 operates as described, so that this burst of signal appears on a vertical blanking pulse, preferably on the portion of this pulse following the vertical synchronizing pulse.

It is necessary to distribute the key signal pattern recorded on the code tape 21 of the system or Figure l, or the patterns on the code tapes 21a, 21h, 21C of the modification shown in Figure 1A, to the subscriber receivers. This distribution may 'ce conveniently accomplished by suitable switching apparatus at the transmitter, or by a distributing station located near a group of subscriber receivers. Such a distributing station is shown in Figure 6, wherein line circuits S2 a,`32b,32c kextending from.,the transmitter.

The frequency multiplied vertical are brought into the central station as slivvii,` The line circuits 32a, 32h, 32e respectively carryl recurring key signal patterns corresponding to the patterns on the respective tapes 21a, 21h, 2`|c of Figure 1A; and these circuits are connected to selector switches I58a, |581), and others when so desired. The selector switches |58al and |581) are connected respectively to switching devices |560, and |5919, and these devices are in turn respectively coupled to telephone head-sets |60a, |6622. The coupling to the head-sets may be accomplished by inductive coupling between coils |6|a and |6|b and the respective voice coils in the head-sets, or by other suitable means. The telephones are connected to subscriber receivers over the usual telephone networks and connection is made to these networks by lines |62a, |62b.

The switching devices |59a and |59b are connected to a pick-up head |63 associated with a tape |64, through respective switches |65a and |655. A permanent signal is recorded on tape |64 by any known means, and this signal is impressed on devices |59a or |5621 whenever switches |6511 or |651) are closed. The devices |59a and |591) are similar to the switch device I6 of Figures 1 and 3, and these devices close the connection from selector switches |58a and |581) to coils |6|a and |6|b whenever the sig nal from tape |64 is impressed thereon.- The tape |64 is synchronized with the tapes 26, 21 and 26 of Figure 1, and is driven at a speed that is some sub-multiple of the speed of tapes 26 and 21, for example, at one revolution per minute. Synchronization between the tape |64 and the transmitter tapes is eiective by means of a television receiver |66. This receiver is coupled to a suitable antenna and tuned to the television signal transmitted by the system of Figure l. Vertical synchronizing pulses are separated from the received television signal and applied to a generator |61 which drives a synchronous motor |66, which in turn drives the tape |64 at the required speed. The length of the signal recorded on tape |64 is made such that when this tape is revolved at the required speed and switch |65a is closed, device |59a is caused to close for a time interval corresponding to one revolution of the code tape 21 of Figure l. Similarly, when switch |6517 is closed, device |59b is caused for a time interval corresponding to one revolution of the code tape 21.

Hence, sometime before the program intervals of the transmitter of Figure 1, a subscriber may receive key signal patterns to decode the television signal for one or more program intervals, whatever he desires. To do this the subscriber merely calls the central station, this call being received for example over the line |62a on a telephone |66a. An operator answers his call and instructs him to erase all signals from the tapes of his receiver (the receiver tapes and apparatus associated therewith are to be described herein at a later point). The operator then requests the subscriber to switch in the first receiver tape, and the selector switch is then moved to select the line circuit 32a. Just prior to the coincidence of the leading edge of the signal on tape |64 with the pick-up head |63, the operator closes switch |6511, and leaves this switch closed until the trailing edge of the signal on tape |64 has passed the head |63, and then switch |65a is opened. In this manner, a

single occurrence of the recurring key signal pat-f tern on line 32a is transmitted to the-subscriber receiver where it is recorded von a receiver tape.,

The -fpei'ato'r1iti-ien instructs thesubscriber Ito switch intiisiseoondltape, `anditurnsthe selector switch v- |-58a'1' to select line-L 32hl and the operation islrepeated. By similar-operations, the-single occurrence of the key signal= patternony line 32o may' befl transmitted toi thev subscriber receiver; to loeiecoidedonA the third receiver tape.

Y lHencefthe subscriberfreceiver may obtain one, two orthree keysignalpatterns; each of which conditions his'receiverto decode the television signals fora-certain--program interval. It is pointedout thatisuch conditioning ofi' thesubscriber receivers may bepa'rr-idf-out-several-hours before the progra-m inter-vals, -so/thatlat the beginning of the mst-program interval, ali-thesubscribersdesiring-tofreceive the ensuingA programs'are equipped withthe'necessarykeyfsignal patterns.

""I-lie'L Whole operation of distributing the* key signabfpatterns "'-to -a subscriber receiver takes place in 'a' relatively f short-,i interval-before 'the commencement-lofi the-program interval and each subscribennses the -itelephone network only for the yshort interval-i'beforethe program interval aridi not continuously, as in previously disclosed systems?. Ililla-nv subscribersmay be servedover the'telephone head-sets' ISDw-and |5019, and additional headset'sl mayv be installed -inthe central stationilwhenithef'number of 'subscriber' receivers in the locality: off the'fstation 'warrants such installaton.

"he-apparatns* of the Ycentral station has been -describediaslbeing'manually controlledlbyan: operator. fHoweverywhensodesired'automatic control mean-s may *be f -usedrf to effect the receiver switching operationsin response to a request for the key'signal patterns byfa subscriber.

v`AY subscriber receiver for receiving and decoding 'the `ccrnipos'ite-.-television -signal `from the transmitter'of?Figurefl-is shown'inFi-gure 7. 'The recevecoompri'ses atuner anddetector '|69which may.: be coupled to any suitable antenna y l 10. The tuner vand detector' is connected to a video ampliiier .|11 :Lwliichfin turn is connected to =a usual limageitube lor reproducing device |72. Synchronizingfsignalzcomponents inthe received televisionvv signal* are separated therefrom bymeansf offafsynchronizing. signalv separator |13Y c/oupledf-lto-` theloutpu-tterminals of `detector- |69 airidhiurther ooupl'ed'ftofa ver-tical sweepgenerator |14, andA ftog-f-ahorizontal sweep .generator H9 through a delay line and switch |15. The veriticall'fa'aid horizontal sweep 'generators are respectivel .connected tothe vertical sweep coilsv |11` andA vhorizon-tale sweep .coils 18 associated with tubeV |1142.. ff-Ifhe' delay llin'e yand 'swit'ch '|15 iis' controlle'di-by-ailceylsignal filter and switch operator 1 1coupled-thereto by 4means vkof three Ileads i|F8 0. 'Ilhef'sw'fitch operator |19 is also connected tothe verticalfsw'eepgenerator I'HlA by meansfo'f'lleads |81., andi'toa-pick-up head l2lassociatedfwith-a receiver :tape f! S3 by1way-oi` leads ifi..

In the operation 4ofthe "receiver .described to this. pointr 'the coded' television i signalffroin vthe system. ofzlligure: 14 fisfre'c'eived on antenna ll'lfu, andtunedand detected :inreceiver stages 4169-. The video-frequency.components `tof the `detected signalfarei amplified in theiamplier .L1-rl V:and fare then impressedmpon :the: reproducing @device .f1.2 in:thexusualsmanner, toc'ontrolidzhe intensityvof the cathodefray lbeam finfcthis tube. They-synchronizing-signalfcomponents.oi the detected sig'- nali-:are separated therefrom in ftheirseparator stage :|=1:3'.2.and1 the verticalv `.synchronizing.' pulses areuse tofsynchnonizethe"vertical sweepsgenerato: f|.111|fwvithI. the received signal, 'fll'henhor-n thereof.

zontal synchronizing-pulses' areapplie'd lto the' horizontalV sweep generator Y |161 through-.a .delay line-and -switch |15', and `this delay `line and switchioperates in-two positions, in amanner similiar to fthecorresponding stage H3 vof Ythe transmitter'vof Figure 1, to pass thefhorizontal synchronizing pulses undelayedin one position .s leads iuand'vertical .blankingpulses from the vertical .sweep .generator I'I'tlron. leads i 8 vThe keysignallter and switch operator is sharply tuned to'beiresponsive to signals on leads,l |94 ofthe'frequency of each key signal burstandto discriminate against other signal frequencies. Asy fully vdescribed in copending .application Serial No. :773,848, when .the key signals on-leads it arexproperly timed with the key signals atthe transmitter, delay line and switch |75 is operated in synchronism with the delay line and switch lfdfofcFigure 1'. Hence, delay line and switch l'lfactsr to. delay the horizontal scanning of the irnagetube |12 duringthespaced intervals when thevicleo-frequency components vof the lreceived television signal are delayed, and `tlfierefor acts to compensate `for the delay and thus to decode thereceivedltelevision-signal. Under these conditions, image tube |12 reproduces intelligence contained: in the received television signal.

Inrcontrast with the previously disclosed systems, the key signals are not impressed directly on .the leads |841over=a line circuit extending to the' transmitter. In .the present system, to receive the key-signal thesu'bscriber calls the central station -of Figure by means of a telephone head-setfm which is connected -to the usual telephoneHn-etwork by a line |86. A recording coil |81-.associated with tape |93 is shown coupled to fthe` line |86 by -lway of a switch |88 and Van i inductancercoil ISB-coupledA to the voice coil in col the head set |85,falthoug.h any known coupling or connectingmeans between the recording coil tflrandlthe line |86 may be used. An erasing signal-source l89 .is connected through'a switch |99 to yan erasing -head |9|- associated with the tape |83. After connection has been -made with the central station, the subscriber is requested toeraseaan-y existing r'signal from the tape |83, and. .to do this -he closes `switch :|99 for several revolutions of the tape. When the `erasing is ycompleted-.and:switch f| is opened, a single keysignal patternV is .transmitted over the line circuit from .the 'centralstation as previouslythe synchronizing signal: separator |13 to amultivibrator H192. This' causes the multivibrator`-|92 to` fbe'isyn'chronized with fthe vertical synchronizing pulses ofthe; .received ysubscription tele vision signal, and :this multivibrator synchronzes argenerator I.-9.6. at.'this frequency. "lhe'genera-` tor |96 in turn drives the motor |89 at this synchronous frequency, and therefore when the switch 268 is closed the receiver tape |83 revolves at a speed identical with that of the transmitting code tape 27 of Figure 1. Hence, the key signal pattern recorded on the receiver tape |83 is identical to the key signal pattern on the transmitter code tape 27 of Figure 1.

When the recording operation at the receiver is completed, the switch 280 is opened, the switch |88' is opened, and the motor |89 is synchronized in the following manner: Vertical synchronizing signals from the separator |73 are impressed on the multivibrator circuit |92 by way of leads |93, a switch device |98, and leads |95. the switch device |94 is closed the multivibrator |92 is synchronized at the frequency of the vertical synchronizing pulses. The multivibrator |92 in turn synchronizes a generator |98 at this frequency, and the generator drives the synchronous motor |89' at the required speed so that the receiver tape |83 revolves at the same rate as the code tape 27 of Figure l.

Since identical key signal patterns are recorded on the transmitter and receiver tapes, and since these tapes are rotated at the same speed, as long as these patterns are in phase with each other, the television signal is coded at the transmitter and decoded at the receiver, and device |72 reproduces an undistorted television image. However, should receiver tape |83 fall out of step with the code tape 27 of Figure 1 due to the receiver being shut off for a certain period, or for other reasons, the key signal pattern recorded on the receiver tape is no longer in phase with the key signal pattern recorded on the transmitter code tape and, hence, the receiver is no longer able to decode the coded signals. For this reason, means are provided for bringing the receiver tape back into step with the transmitter tape, and this is ,accomplished in a manner to be described. The control signal from control tape 29 cf Figure 1 is received over line |88 with the key signal pattern, and this control signal is recorded on tape |83 together with the key signal pattern, and applied to the switch device |98 over leads |97. This control signal is also applied to switch operator |79 over leads |84, but as previously described the control signal has a different fref quency than the key signal and the switch operator |79 discriminates against and is unresponsive to this control signal. Similiarly, the key signal is also applied to switch device |94 over leads |97, but the switch device |99 discriminates against all frequencies except that of the control signal. The equalizing pulses of multiplied frequency from control signal phasing circuit 4| of Figure 1 are selected from the received television signal by a conventional lter and rectifier |98, and the rectified output of lter and rectifier |98 is also impressed on switching device |98. A single time coincidence of a signal from filter and rectifier |98, and the control signal from leads |97, causes the switching device |98 to close and remain closed, thus passing the vertical synchronizing signals from leads |93 to leads |95 and hence to multivibrator |92. The multivibrator |92 is given a free-running rate which is slightly lower than the frequency of the vertical synchronizing signals. When the receiver tape |83 is out of step with the transmitter code tape 27 of Figure 1, the subscriber depresses a push button, not shown, on switch device |94 which opens this device until the next When coincidence of the control signal and signal from rectifier |98. When device |94 is open, multivibrator |92 oscillates at its free-running rate and synchronizes generator |98 at a slightly lower frequency than that of the vertical synchronizing signals, and therefore tape |83 turns at a slightly lower speed. When the control signal on tape |83 aligns itself with the control signal on tape 28 of Figure 1, the signal from lter and rectifier |98 and the control signal from tape |83 occur simultaneously at switching device l9, and this simultaneous occurrence causes the switching device |94 to close and remain closed, thus passing the vertical synchronizing signals to the multivibrator |92, and the tape |83 immediately locks-in with the transmitter code tape. The receiver is, therefore, again conditioned to decode the received coded signals. As previously stated, switching device |98 is so arranged that when the control signal from tape |83 and the signal from the filter and rectifier |98 occur simultaneously, the switching device |94 closes and immediately passes vertical synchronizing pulses to the multivibrator |92, and continues to pass these synchronizing pulses. Hence, when the receiver tape |83 loses synchronism with the transmitter tape, the operator merely depresses the above mentioned push-button and in a short interval synchronism is restored.

The pick-up and recording head |82 may be made adjustable in a direction along the longitudinal axis of the tape |83, as indicated by the arrow |99. This allows for a slight adjustment to be made to compensate for time delays in the lines and circuits used in the transmission of the key signal pattern to the receiver.

In the receiver of Figure '7 as in the transmitter of Figure 1 a single code tape is used and the setting up of the pattern on the tape must be done sometime before the program interval. When, as indicated in Figure 1A a plurality of code tapes is used at the transmitter, the key signal pattern may be changed between program intervals by merely switching out one code tape and switching in another. To enable the receiver to decode the television signal when the key signal pattern is so changed at the transmitter, the receiver may be equipped with a plurality of tapes, so that different tapes may be switched into the receiver circuit to correspond with the changes at the transmitter. Such a system for the receiver circuit is shown in Figure 7A.

In the system of Figure 7A, the synchronous motor |89' drives a plurality of receiver tapes |83a, |832), |830. Any time before the program intervals the subscriber may call the central station and receive the required patterns and,

.. upon operation of a selector switch 288, may

record these patterns respectively on the three tapes. The subscriber may close switch 28m and use the pattern on tape |83a for the first program interval, and likewise may selectively close switches 28|?) and 28| c for following program intervals. In each case the tape selected must have a key signal pattern recorded thereon which coincides with the key signal pattern used by the transmitter for that particular interval. Moreover, the change over from any one control tape to another may be achieved automatically by any conventional time controlled selector.

It is pointed out that with the modified systems of Figures 1A and 7A the transmitter may provide three or more patterns for three or more separate intervals, and the subscriber has the option of purchasing one or more of these patterns, depending on the number of programs that he Wishes to receive.

Switching device |94 is shown in detail in Figure 8, and referring to this figure, the input terminals of the circuit are designated 292, and these terminals are connected to the leads |91 and hence to pick-up coil |82 of Figure '7. One oi.' the terminals 292 is grounded and the other is coupled to av control electrode 293 of an electron discharge device 294 through a coupling capacitor 295, control electrode 293 being connected to ground through a grid leak resistor 296. The cathode 291 of device 294 is connected directly to ground, and the anode 209 of this device is connected to the positive terminal of a source 299 of unidirectional potential through a tuned circuit 2|0, the negative terminal of source 299 being grounded. The rtuned circuit 2I0 consists of an inductor 2H shunted by a capacitor 2| 2, this circuit being tuned to be resonant at the frequency of the control signal on tape |93 of Figure '7, so that this signal is amplified by the circuit or device |94, and all other signals are discriminated against.

A rectifying device 2 I3 has its anode connected to anode 299 of device 294 and its cathode connected to a control electrode 2|4 oi' an electron discharge device 2|5, the common junction between the cathode of device 2 I3 and control electrode 2 I4 of device 2 I 5 being connected to ground through a resistor 2|6, and this resistor being shunted by a capacitor 2 I1. The device 2 I3 conducts until capacitor 2|1 is charged, and i electrode 222 of device 2I5 is connected to the l positive terminal of source 299 through a resistor 223, and this electrode is' coupled to ground through a capacitor 224. The rectified equalizing pulses of multiplied frequency from iilter and rectifier I9 of Figure '1v are impressed across terminals 225, one of these terminals being connected to ground, and the other being coupled to a second control electrode 226 of device 255 through a capacitor 221. Control electrode 226 is connected to ground through a grid leak resistor 228. The anode 229 of device 2I5 is connected to the positive terminal of source 299 through a load resistor 230. The device 2I5 is normally biased to cut-off by the positive potential on cathode 2I8 due to the potential divider action of resistors 22| and 2|9, and the circuit of this device is so arranged that the device becomes conductive only when the control electrodes 2 I4 and 22E are simultaneously driven positive. As previously described, this condition occurs only when the control signal on control tape 26 of Figure l and the control signal on the receiver tape |83 of Figure '1 occur simultaneously, that is, when ther tapes at the transmitter and at the receiver are in step.

The anode 229 of device 2 I5 is connected to a control electrode 23| of an electron discharge device 232, this control electrode beingconnected through a resistor 233 to the negative terminal 29' of a biasing source 234, the positive terminal'of this source being grounded. Cathode 235 of device 232 is connected to ground, and the anode 236 of this device is connected to the positive terminal of source 209 through a resistor 231. The anode 236 of device 232 is further connected to the control electrode 239 of an electron discharge device 239 through a resistor 249, this control electrode being connected to the negative terminal of source 234 through a resistor 24|', and this resistor being shunted by a push-button 242. Cathode 243 of device 239 is connected to ground, and the anode 244 thereof is connected to the positive terminal of source 299 through a resistor 245. The anode 244 of device 239 is further connected to the control electrode 23| of device 232 through a resistor 249. The control electrode 238 of device 239 is further connected to the control electrode 24?' of another electron discharge device 248. The cathode 249 of device 248 is connected to ground through a resistor 250, and the anode 25| of this device is connected to the positive terminal of source 299 through a resistor 252. Vertical synchronizing signals from synchronizing signal separator |13 of Figure 7 are impressed across terminals 253, one of these terminals being grounded and the other being connected to cathode 249 of device 248. When device 248 is conductive, the vertical synchronizing signals impressed across terminals 253 appear across output terminals 2 54, one of the terminals 254 being grounded and the other being connected to anode 25| oi' device 248. Terminals 254 are connected to the multivibrator |92 of Figure 7 by way of leads |95.

In normal operation, the value of the negative potential of source 234 is made suflicient to overcome theV positive bias on control electrode 23| and drive this control electrode negative, and hence device 232 isnon-conductive and the relatively high potential value of anode 236 impresses a positive potential on control electrode 239 of device 239, by reason of the connection of anode 236 to control electrode 238 through resistor 249. This positive potential overcomes the negative` bias on control electrode 233 trom source 234, and causes device 239 to be conductive. Control electrode 233 of device 239 is connected to control electrode 241 of device 248, and hence in this condition control electrode 241 has a potential impressed thereon sufciently positive to make device 248\conductive. Hence, vertical synchronizing signals impressed across terminals 253 are conducted by device 249 and appear across terminals 254. Therefore, vertical synchronizing signals from the synchronizing signal separator |13 of Figure '7 are passed by the switching device |94 to the multivibrator |92, and the receiver tape |83 is rotated in synchronism with tape 21 of Figure l. Should synchronism between the transmitter and receiver tapes be lost, it is merely necessary for the operator to press push-button 242 for a moment and then release this button. When push-button 242 is depressed, negative biasing potential from source 234 is applied directly to control electrode 238 of device 239. This potential is' sufficient to drive device 239 to cut-olf, and the resulting rise in anode potential of device 239 overcomes the negative bias on control electrode 23| of device 232. Device 232, therefore, becomes highly conductive and holds device 239 in a non-conductive state. Hence, a momentary depression of push button 242 causes device 239 to become non-conductive and device 232 to become conductive. 

